Polypropylene in its many and varied forms is a long establish staple of the polymer industry. Depending upon its form, it exhibits a number of desirable properties including toughness (as measured by any of a number of impact tests, e.g., notched Izod, dart drop, etc.), stiffness (as measured by any of a number of modulus tests e.g., Young's), clarity, chemical resistance and heat resistance. Often a particular combination of properties is desired that requires a balancing of various properties against one another (e.g., stiffness against toughness). For applications that require good processing characteristics (e.g., fibers), preferably the polypropylene has a relatively narrow polydisperity or molecular weight distribution (MWD), e.g., less than 3.5.
Many forms of fibers and fabrics have been made from thermoplastics including, as noted above, polypropylene. The properties of the fibers and fabrics are a function, at least in part, of the polymer(s) from and the process(es) by which they are made. Representative of these various polymers, fiber and fabric types, and processes for making the fibers and fabrics are those described in U.S. Pat. Nos. 4,076,698, 4,644,045, 4,830,907, 4,909,975, 4,578,414, 4,842,922, 4,990,204, 5,112,686, 5,322,728, 4,425,393, 5,068,141 and 6,190,768.
Crystalline polypropylene has an isotactic structure, and it is readily produced using a Ziegler-Natta (Z-N) or a metallocene catalyst. While metallocene catalysts are effective for producing propylene homo- and copolymers with a high isotactic index and a relatively narrow MWD, to produce high Mw, e.g., over 300,000, propylene homo- or copolymers economically with a metallocene catalyst is relatively difficult, especially in a solution process. Moreover, the industry maintains a continuing interest in new polypropylene polymers, particularly those for use in fiber applications.